Buckling-restrained braces are currently used, as a new type of energy dissipation-seismic reduction members, in civil architectural structures, especially in multistoried tall buildings. The buckling-restrained brace provides lateral stiffness to the structure during a small earthquake, and tends to dissipate earthquake energy during a moderate or large earthquake. In practical application, the buckling-restrained brace must be connected to beam and column by a gusset plate. FIG. 1 is a structural diagram showing the overall connection between a buckling-restrained brace and a beam or column. FIG. 2 is a structurally schematic diagram of a traditional gusset plate connection for buckling-restrained brace. As shown in FIGS. 1 and 2, in the traditional connection method, the gusset plate is connected to flanges of the steel beam and column by means of fillet welds, wherein the gusset plate for buckling-restrained brace comprises a central plate 1, a first rib plate and a second rib plate. Such connection type has the following problems:
1. Problem of Rigid Zone at Brace-Beam-Column Joints
Since the gusset plate has large in-plane stiffness, the rotational stiffness of a beam-column joint may be significantly increased, resulting in the formation of a rigid zone effect at the beam-column joint. The currently existing tests and finite element analysis results show that such rigid zone effect may significantly increase the lateral stiffness of the main structure and result in an increase of the seismic shear force sustained by the structure, thereby causing premature failure of the structure (see the second point for details), which in turn weakens the advantageous effects that the buckling-restrained braced structure should possess. However, during the design of structures, engineers often neglect the impacts of this rigid zone effect due to the problems such as complication of computer modeling, which will result in a large deviation between the actual performance and the designed performance of the buckling-restrained braced structure system, leading to potential man-made risk to the safety of such structures when subjected to a large earthquake.
2. Transfer of Failure Mode of the Beam-Column and Change in the Effective Length Thereof
The results of tests for the buckling-restrained braced frames existing in China and abroad show that the rigid zone effect may cause the transfer of yielding (failure) section of the beam-column from the end of the beam to the end of the gusset plate, and cause a peak value of plastic deformation thereof to be 32 times of that of a model without a rigid zone. In addition, the rigid zone effect may decrease an effective length of the beam-column, cause the failure mode of the beam-column to be transferred from bending failure to shear failure, and aggravate the tendency of brittle failure in the reinforced concrete beam-column member, which will be fatal in a large earthquake.
3. Adverse Impact of the Opening-Closing Effect of the Beam-Column on the Gusset Plate
The studies show that the beam and column will experience significant bending deformation (the flanges thereof are subjected to elongating or compressive deformation) under the seismic shear force. But the rigid zone effect of the gusset plate restrains the beam and column from deforming freely. An additional acting force (termed Opening-Closing effect) is thus generated at the connection welds between the gusset plate and the flanges of the beam and column, causing premature fracture of the connection weld of the gusset plate, and even leading to out-of-plane buckling failure of the gusset plate in the case that the brace is tensioned. However, such Opening-Closing effect has not been fully taken into account in the prior gusset plate designing methods, so that the connection of the gusset plate in turn becomes a vulnerable spot, which is contrary to “strong connection, weak member” principle of seismic design, and cannot guarantee the full exertion of the energy-dissipating capability of the buckling-restrained braces.
In view of the above, in an earthquake, especially a large earthquake, significant in-plane interaction effect will be present between a structure and a gusset plate, which will adversely impact the seismic performance of the structure and the connection performance of the gusset plate. Moreover, it remains difficult to provide full consideration to the adverse impacts in the design of the structure, and it is impossible to quantitatively guarantee the safety of the buckling-restrained brace structure under a large earthquake or major earthquake.